An accelerometer is one of the major sensors used in navigational systems, particularly inertial navigational systems, and on-board automotive safety control systems. Automotive examples of accelerometer use include various anti-lock braking systems, active suspension systems, and seat belt lock-up systems.
Generally, in a typical accelerometer, a mass is suspended by two springs attached to opposite sides of the mass. The mass is maintained in a neutral position so long as the system is at rest or is in motion at a constant velocity. When the system undergoes a change in velocity in the direction of the springs' axis, and therefore is accelerated in that direction, the spring mounted mass will at first resist the movement because of its inertia. Therefore, this resistance to the movement or delay, will force one of the springs to be stretched and the second to be compressed. The force acting on each spring is equal, yet opposite, in magnitude and is equal to the product of the weight of the mass and the acceleration of the mass. The acceleration which is determined by the accelerometer is the change in velocity experienced by the mass.
Microaccelerometers having resonant microbridges are known. Examples of this type of accelerometer are disclosed in U.S. Pat. No. 4,805,456 issued Feb. 21, 1989 to Howe et al. entitled, "Resonant Accelerometer," and U.S. Ser. No. 274,180 filed Nov. 21, 1988 to Chang et al. entitled, "Resonant Bridge Two-Axis Microaccelerometer." In a microaccelerometer of this type, a proof mass is suspended by at least one pair of resonant microbridges. The proof mass is generally formed using micromachining techniques. The pair of resonant microbridges are attached to opposite ends of the proof mass along a common axis. This type of resonant microaccelerometer is attractive for precision measurements, because the frequency of a micromechanical resonant structure can be made highly sensitive to physical signals.
In a microaccelerometer based on resonant microbridges, the acceleration in the plane of the substrate causes differential axial loads on oppositely disposed resonant microbridges, i.e., causes one supporting resonant bridge to be in compression and the other in tension. It is the inertial force on the proof mass which generates the axial load on the resonant microbridges. In turn, the compressive and tensile loads shift the inherent resonant frequencies for each resonant microbridge. The difference between the resonant frequencies of the compressive and tensile members is measured and used to determine the magnitude of the acceleration component in the direction of the common axis shared by the pair of resonant microbridges.
It is desirable to provide a method for forming these types of resonant bridge microaccelerometers, in particular without the use of cumbersome and deleterious micromachining techniques, such that the microaccelerometers are reliable, accurate, rugged, and low cost, all characteristics which are required for on-board automotive systems, as well as other applications.